U.S. Pat. No. 3,013,905 teaches a transducer which includes a magnet plate and a membrane. The magnetic plate is made from highly coercive oriented ferrite material, e.g. the barium ferrite commercially known as xe2x80x9cIndox Vxe2x80x9d of a high coercive force. The high coercive force is of the order of 2000 oersteds. It is magnetized in such a manner that alternating north poles and south poles extend in parallel over the entire length of the magnetic plate. Between each of two vicinal poles the flux runs through the depth of the magnetic plate. The flux can be conceived as a horseshoe magnet. The membrane is a pliable sheet of non-magnetic material, such as a polyester plastic material, of a thickness of about 0.01 millimeter. On it, a conductor of a material, such as aluminum, is printed in the form of a very thin, flat band. The thin, flat band is pliable and has a very low mechanical impedance. The membrane is substantially coextensive with the magnetic plate, tautly stretched above the plate at a distance of about 2.0 millimeters or less and secured at its edges in any suitable conventional manner. The conductor is continuous and runs in parallel stretches from end to end of the membrane, returning at the ends in short arcs. The stretches are in registry with the magnetic gaps between consecutive opposite poles of the magnetic plate with the gap between the poles of the magnet and the stretch with the gap between the poles of magnet. The expression of magnetic gaps does not imply a conventional air gap as the magnetic plate has a stretch that is a continuous plane surface. At its ends the conductor has two or more terminals for connection to the input or output circuit, as the case may be. The magnetic plate has a plurality of holes, for the equalization of the air pressure in the gap between the magnet plate and membrane. When an electric current flows in the conductor, its direction is reversed from stretch to stretch of the conductor. Each change of direction corresponds to a change of direction of the magnetic field or, in other words, the vector product of the current with the magnetic field has the same sign in all parts of the conductor. The membrane thus oscillates in phase over its entire surface with the frequency of the alternating current passing through the conductor. The magnetic plate is built up from discrete bars mounted in parallel on a soft-iron, perforated armature plate 4a with equal gaps between them. Their top faces form alternately north and south poles.
U.S. Pat. No. 4,484,037 teaches a ribbon-type electro-acoustic transducer which has a magnetic system. The magnetic system includes an upper plate and a center pole between which an air gap is formed. A diaphragm on which conductors are arranged is disposed in the air gap. The upper plate includes two plate-shaped parts between which a space is formed in which an edge portion of the diaphragm is located. This results in a more homogeneous magnetic field so that the transducer distortion may be reduced. Moreover, the transducer sensitivity is improved and is suitable for handling signals in the mid-range audio frequency spectrum. The cavity enclosed by the magnet system and the diaphragm can be acoustically coupled, be via an additional cavity to a bass-reflex duct or an additional passive radiator diaphragm.
U.S. Pat. No. 5,850,461 teaches a diaphragm mounting system for flat acoustic planar magnetic and electrostatic transducers. The system incorporates opposing frame sections. Each frame section defines a clamping or peripheral surface area and an internal or central area through which acoustic waves may pass from the diaphragm. The diaphragm is first placed on one frame section with zero plus tension. The second frame section includes a protruding ridge extending substantially along an inner edge of the central area which ridge defines a border for a sound producing area of the diaphragm. During assembly of the two frame sections, the ridge engages the diaphragm to place predetermined tension on the diaphragm as the sections are joined. The profile of the ridge may be shaped to provide predetermined biaxial tension in a diaphragm of generally rectangular shape.
U.S. Pat. No. 4,471,172 teaches a planar diaphragm type magnetic transducer with magnetic circuit in which the magnet strips on the soft iron plate and confronting the diaphragm are arranged in a sequence south, north, north, south, south, north, north, south, et seq. The magnet strips are spaced across the transducer and the metal plates on which the magnet strips lie are apertured to make the plates acoustically transparent. Conductors are grouped in runs on the diaphragm opposite alternate pairs of magnet strips. The magnet strips have magnetic poles of opposite polarity at their front faces.
U.S. Pat. No. 6,104,825 teaches a planar magnetic transducer that includes a clamping frame, a diaphragm with an electrical conductor and a plurality of magnetic bars. The diaphragm is secured to the frame and has an active surface area under tension spaced inwardly of the frame. The electrical conductor is disposed on the active surface area of the diaphragm. The magnetic bars are mounted so that they are spaced from said diaphragm.
The inventor hereby incorporates the above patents by reference.
The present invention is directed to a transducer. The transducer includes a diaphragm with areas of multiple electrical conductors, two rows of magnetic bars, two metal plates and a clamping frame. The diaphragm is clamped in the clamping frame and is positioned between the two rows of magnetic bars. Each row of magnetic plates is in close proximity to the clamped diaphragm. Each metal plate has holes. The holes correspond to spacing areas between the magnetic bars and acoustically connect the diaphragm to outside media. The magnetic bars are sequentially located on the metal plates with spacing between the magnetic bars. The diaphragm is secured to the clamping frame and has an active surface area under tension spaced inwardly of the clamping frame.
In a first aspect of the invention there are no side magnetic bars at the ends of each of the row. Absorbing strips are used in place of the absent magnetic bars filling the space between the side magnetic bars and the clamping frame.
In a second aspect of the invention the planar magnetic transducer can be made at a reduced cost while retaining the low frequency extension and most of the efficiency. Efficiency is reduced in some points across the reproduced band but not to an extent that is proportional to the magnetic field reduction.
In a third aspect of the invention the planar magnetic transducer provides wider dispersion in horizontal plane.
In a fourth aspect of the invention the planar magnetic transducer provides smoother frequency response and reduced parasitic noise and buzz due to introducing resistive acoustic loading at the periphery of the diaphragm.
Other aspects and many of the attendant advantages will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawing in which like reference symbols designate like parts throughout the figures.
The features of the present invention which are believed to be novel are set forth with particularity in the appended claims.